RFID systems comprise an RFID read/write unit (reader) and electronic tags. The latter can operate passively, i.e. without a battery, and are therefore reliant on the permanent presence of a carrier signal transmitted by the RFID read/write unit, also called a power carrier. The carrier signal simultaneously serves as a radio frequency oscillator for the tag. Although semi-passive tags have a battery, they likewise require a permanently transmitted carrier signal instead of a radio frequency oscillator for modulation. In this context, RFID systems with ranges of several meters use UHF frequencies or microwave frequencies. The RFID read/write unit itself comprises a baseband part and a radio frequency part (RF part) having a transmitter and a receiver.
To achieve reading distances in the region of several meters for passive electronic tags and in the region of several tens of meters for semi-passive electronic tags, a transmitting signal provided by a transmitter (TX) needs to be produced and emitted at a power of approximately 1 watt (30 dBm). A significant part of this comparatively powerful transmitting signal is then injected spuriously directly into the receiver (RX), see FIG. 1, the coupling paths 28, 29 and the reflection at a reflector 27. On the other hand, the receivers in the RFID read/write units need to detect the low levels of the response signal which is reflected unamplified from the tags only after modulation of the carrier signal with response data. Such a resultant, required wide dynamic range in the receiver in the RFID read/write unit represents an enormous demand on any bidirectionally operating system. In this case, the RFID read/write unit can either be operated using a single antenna, with the transmitter and the receiver being decoupled by a circulator, or two directly adjacent antennas are used. In both cases, the isolation between the transmitter and the receiver is known to be low; typically, such isolation is merely around 20 dB.
The transmitting signal injected directly from the transmitter into the receiver is therefore not only very powerful but also unwanted, since it results in intermodulation with the response signal from an electronic tag in the radio frequency stages of the receiver and hence reduces the sensitivity of the receiver. In other words, the isolation from the transmitter (TX) to the receiver (RX) for an RFID read/write unit is much too low for long ranges in practice in the UHF and microwave domain. The isolation is determined by the design of the RFID read/write unit and particularly by the technology of the circulator at the antenna output or by the arrangement of the transmission and reception antennas used. The use of a circulator can subsequently be handled in the same way as the use of separate antennas. The latter is discussed here as representative of both uses.
A numerical example demonstrates the problem explained above as follows: if the transmitting power is 30 dBm with isolation of 20 dB, this results in a spurious signal of +10 dBm at the receiver input. The useful signal emitted by a passive electronic tag is only just −70 dBm, however, in the UHF domain at a distance of approximately 4 m. The high level of +10 dBm for the transmitting signal injected directly into the receiver thus overdrives the input amplifier of any RFID read/write unit, which is typically a low noise small signal amplifier.
Intermodulation frequencies arise between the injected transmitting signal and the receiving signal from electronic tags or the signals from other simultaneously active RFID read/write units. If the receiving signal is digitized for further evaluation, a dynamic range of 80 dB additionally produces a problem with the available resolution in the case of the 14 to 16 bit analog/digital converters which are usual for this. If one wishes merely to solve the problem by improving the components, the demands on the RF components and the AD converter become very high and unappealing, particularly as far as linearity and drawn supply current are concerned. Rejection of or electronic compensation for the injected transmitting signal is thus necessary in order to achieve long ranges.
A known reception architecture therefore provides what is known as direct conversion stage (DCS) in order to move from the radio frequency (RF) to a baseband. If the data on the electronic tag are modulated and reflected without a direct-current voltage component then the dynamic range for the AD converter can be alleviated by filtering away the direct voltage component (DC) after down conversion in the receiver's DCS. In the RF input part of a receiver, however, nothing changes about the intermodulation situation, and electronic compensation continues to be required. Since removing the DC voltage component eliminates the contribution of the injected transmitting signal in baseband, said contribution needs to be detected and processed in an additional circuit. In practice, a maximum receiving signal with a level of −10 dBm for example, would be desirable. On the basis of the above calculation, an isolation of at least 20 dB is therefore additionally necessary.
US 2004/0106381 and U.S. Pat. No. 6,229,992 B1 propose measuring the injected signal in the receiver, comprising a reception antenna 11, an additional stage 8, an RX converter 4 and an AD converter 2 in FIG. 2, and adding a compensation signal, which is derived directly from the transmitting signal in the RF part, in the reception path shown.
As is known, a read unit 19 comprises a software defined baseband part 1 (SDR) and an RF part 18, as shown in FIG. 2. In such SDR based transmission/reception installations, the complex value signals are generated respectively processed purely by computational means in a signal processor 6 to the extent that they now need merely be shifted by means of linear converters (up converter or down converter) to, or from, the radio frequency band (RF band). A TX converter 5 in the transmitter is fed with a complex baseband signal (inphase and quadrature signal) which is output by the signal processor 6 via a dual digital/analog (D/A) converter 3. The output signal is forwarded to a transmission antenna 12 via a directional coupler 7. From a reception antenna 11, the receiving signals are converted into a complex baseband signal (inphase and quadrature signal) by means of an RX converter 4 and are forwarded to a dual A/D converter 2 and accepted by the signal processor 6. In the conventional solution, the transmitting signal is obtained from the output RF signal from the directional coupler 7 and is weighted in the vector modulator 10 with the correction values for phase and amplitude by the DSP 6 using a slow D/A converter 9 and is supplied to the receiving signal in an addition stage 8 for the purpose of compensation.
In a first step of the reading operation, the receiving signal is usually digitized and analyzed over a time interval T0 in a brief Listen Before Talk Phase (LBT phase) of a reading request cycle while the transmitter in a first RFID read/write unit is turned off. This signal contains the request signals from further RFID read/write units, and it is possible to decide whether or not the transmitter in the first RFID read/write unit can be turned on. If the first RFID read/write unit is switched to transmission after the LBT phase, its own receiver must first of all compensate for the injected TX signal in order to achieve a high level of sensitivity.
The compensation signal is obtained from the transmitting signal by adjusting the gain and phase (gain/phase adjuster). This technique is known as adaptive filtering. In this context, the amplitude and phase are adjusted using what is known as the vector modulator 10 totally in the RF domain. The components for outputting the transmitting signal, for addition in the reception path, and also the vector modulator 10 are RF components, however, which themselves have inaccuracies and features which are not ideal. Immediately effective (instantaneous) reduction of the coupling therefore appears possible only with difficulty and in practice by application of iterative cycles of measurement/compensation/measurement, etc.
U.S. Pat. No. 6,229,992 proposes the explicit use of a digital signal processor (DSP) for controlling the rejection, in order to control these cycles. Before each reading operation, a calibration phase is required. If frequency hopping (FH) is used in accordance with the current radio specifications, the calibration needs to be carried out continuously for a number of frequencies. The calibration must additionally be carried out in harmony with the Listen Before Talk (LBT) processes which are to be used according to more recent radio specifications. When these known methods are used, additional loading on the channel arises in any case, and there is a loss of valuable transaction time with electronic tags. U.S. Pat. No. 5,691,978 proposes combining antenna isolation, analog RF rejection and a digital echo canceller in baseband in order to achieve a high level of isolation. The effort for this is considerable and makes no sense in economic terms for an RFID read/write unit.
A common feature of all of these known methods is that, if a receiver designed on a DCS basis is used, it will be necessary to evaluate a DC voltage signal (DC signal) in order to obtain information about the amplitude and phase of the injecting transmitting signal. However, this DC signal is in turn itself subject to errors as a result of cross-coupling effects in the RF mixers in the RX converter. The same applies to methods which measure detection of crosstalk using a conventional envelope detector. Additional problems arise when other RFID read/write units unintentionally transmit on the same frequency channel at the same time, since their transmission frequencies possibly differ only slightly and thus corrupt each measured value. If a transmitting signal from another interfering RFID read/write unit is present then this means that it is sometimes not possible to carry out calibration at all. This problem is either solved by the specification for use of LBT or by synchronizing the entire RFID read/write unit network.